1. Field of the Invention
The invention relates to a battery charging device, more particularly to a battery charging device with fast and slow charging capabilities.
2. Description of the Related Art
Portable electrical devices usually require a battery source so as to operate the same. Rechargeable battery units are preferably used in order to reduce the costs incurred by the consumer. Different types of battery charging devices are presently available to charge the battery units.
In order to reduce the charging time while avoiding damage to the battery units, the battery charging device initially operates in a fast charging state until the battery voltage reaches a predetermined value. The battery charging device then automatically switches to a slow charging state in which current flow to the battery unit is minimized so as to prevent damage to the battery units.
FIG. 1 is a schematic electrical circuit diagram of a conventional battery charging device which can automatically switch from a fast charging state to a slow charging state. A transformer (T) receives an external alternating current (AC) line voltage input and reduces the line voltage input to a more appropriate level. The output of the transformer (T) is received by a bridge rectifier unit which comprises four diodes (D1-D4) and which generates a direct current signal to charge a battery unit (B). The following is a brief description of the operation of the conventional battery charging device:
When the battery charging device is operating in the fast charging state, the direct current signal from the rectifier circuit initially flows through the battery unit (B), a diode (D5) and a resistor (RI), thus charging a capacitor (CI). A zener diode (DZ) is reverse biased and has a voltage which is sufficient to trigger a thyristor (SCR) to conduct. Conduction of the thyristor (SCR) causes a large portion of the direct current signal to flow through the battery unit (B), a temperature switch (T.S.), a current limiting resistor (R3) and the thyristor (SCR). The temperature switch (T.S.) is in contact with the battery unit (B) and monitors the temperature of the battery unit (B). Since the temperature of the battery unit (B) gradually increases when fast charging is being conducted, the switch (T.S.) eventually opens when the temperature of the battery unit (B) reaches a predetermined value, such as 46.degree. C., thereby indicating the completion of the fast charging operation.
The battery charging device conducts a slow charging operation when the switch (T.S.) is in an open circuit state. The direct current signal from the rectifier circuit initially flows through the battery unit (B) and through a current limiting resistor (R5). The resistor (R5) has a resistance (typically 100 .OMEGA.) which is much greater than that of the resistor (R3) (typically less than 100 .OMEGA.), thus permitting the resistor (R5) to reduce the value of the direct current signal when the battery charging device is in the slow charging state.
The above described battery charging device may incorporate indicator units, such as light emitting diodes (not shown), so as to indicate if the battery charging device is operating in the fast charging state or in the slow charging state.
The main disadvantage of the above described conventional battery charging device is as follows: Note that the switch (T.S.) is in an open circuit state only when the temperature of the battery unit (B) is greater than or equal to 46.degree. C. Since the battery unit (B) gradually cools when the slow charging operation is being conducted, the temperature of the battery unit (B) eventually drops below 46.degree. C., thereby causing the switch (T.S.) to close and enable the battery charging device to operate in the fast charging state. The conventional battery charging device is therefore incapable of maintaining the charging operation in the slow charging state when the fast charging operation has been completed.